But Why: A Podcast for Curious Kids Episode Summary: Are Horseshoe Crabs… Crabs? Release Date: May 30, 2025

Introduction

In this captivating episode of But Why: A Podcast for Curious Kids, host Jane Lindholm takes listeners on an exciting field trip to Cape Cod, Massachusetts, alongside Sarah Grady, a Senior Coastal Ecologist with Mass Audubon. The primary focus of their exploration is the enigmatic horseshoe crab—a creature that has roamed the Earth for over 400 million years. Throughout the episode, Jane and Sarah delve deep into understanding why horseshoe crabs are named as such, their unique biology, their role in the ecosystem, and their interactions with humans.

Field Trip to Cape Cod: Observing Horseshoe Crabs

[00:00 - 03:31] Jane Lindholm sets the scene by describing Cape Cod's picturesque beaches, likening the area's shape to a bent arm showcasing bicep muscles. As they arrive at the beach, Jane narrates their first encounter with what initially appear to be large rocks but reveal themselves to be live horseshoe crabs adorned with barnacles and shells.

"We're heading out to a beach not far from where your elbow would be. Just above your elbow, but not near your wrist. It's a little too early to swim. The water is really cold... But it's a beautiful sunny day, and when we arrive it's almost high tide and we can see what look from afar kind of like big rocks in the water." [02:15]

Questions from Kids

Several curious children have submitted questions for this episode, primarily focusing on the nomenclature and nature of horseshoe crabs:

• Benicio (9, Silver Spring, Maryland): "Why are horseshoe crabs called horseshoe crabs?" [03:31]
• Amir (9, Limerick, Ireland): "Why are horseshoe crabs called horseshoe crabs?" [03:31]
• Ava Rose (8, Vancouver, Canada): "Why are horseshoe crabs called horseshoe crabs if they don't look like crabs?" [03:31]
• Additional questions regarding their appearance, behavior, and biology are also presented throughout the episode.

Interview with Dr. Sarah Grady

Sarah's Introduction and Role

Sarah Grady introduces herself as a Senior Coastal Ecologist working with Mass Audubon, specializing in the study of coastal ecosystems and horseshoe crabs.

"We are at a little tucked away beach." [04:15]

Biology of Horseshoe Crabs

Sarah provides an in-depth look at the anatomy of horseshoe crabs:

"The crab has three parts to its body. So it has this front rounded part, it's called the prosoma. And then this part here, this sort of, I don't know, trapezoid shaped part is called the Apistosoma. And it has these little spines on the side. And then this long pointy part, the tail, is called the Telson." [07:11 - 07:40]

Jane further explores the number of claws and their functions:

"They are 10 claws. Their most sensitive part are these flaps here which are their gills." [09:11 - 10:20]

Why They're Called Horseshoe Crabs

Addressing the children's primary question, Sarah explains the origin of their name:

"Well, if you look at the front shape of the horseshoe crab, it looks like a horseshoe. It's a U shape and that's. That's it. Yeah." [07:46 - 08:04]

She further clarifies why they're termed "crabs" despite not being true crabs:

"Most people, I think when they see something that has claws and is walking around in the ocean will think that it is a crab of some sort. Most crabs that you see are going to walk sideways. Horseshoe crabs are walking forward. They are more closely related to spiders and scorpions." [10:20 - 11:05]

Jane reflects humorously on this misnaming:

"Let me just pause here for a second. Did you know that horseshoe crabs aren't really crabs? I didn't. They're more closely related to spiders and scorpions, as she said. So we've been calling them the wrong thing all along. At least we got the horseshoe part right." [11:05 - 11:32]

Survival Through Millions of Years

The discussion shifts to the remarkable longevity of horseshoe crabs:

"They've been around for about 450 million years. Yeah. So they are one of the oldest living creatures walking around here." [11:45 - 11:57]

Sarah attributes their survival to their robust body shape and minimal natural predators:

"They survived by being tough and I think really finding the right body shape. Crabs tend to do really well in general... They really don't have a lot of natural predators when they're in the water, and so they can just keep going." [12:04 - 12:35]

Reproduction and Lifespan

Exploring their mating rituals, Sarah describes how horseshoe crabs lay eggs:

"The female will dig a nest and actually that female is doing that right now. So she's digging in the sand, she's going to lay her eggs and then they're going to move forward a little bit and then the male will fertilize those eggs and in about two weeks those eggs will hatch out." [05:30 - 05:48]

They discuss the impressive number of eggs a single female can lay:

"I read that a female Horseshoe Crab can lay 80 to 100,000 eggs a year." [05:48 - 05:54]

Regarding lifespan:

"So they take about 12 years between the time that they hatch out to the time that they come back to the beach to lay their eggs. And so that means that anything that's done to protect them when they're laying their eggs, we're not going to see if the protections worked really well until those crabs grow up and come back." [28:30 - 29:23]

Sarah estimates their total lifespan to be around 25 to 30 years, noting their molting process slows as they mature:

"They can live up to about 25, 30 years, we think." [30:11 - 30:33]

Interactions with Humans: Consumption and Harvesting

Addressing human interactions, Sarah explains that while humans rarely consume horseshoe crabs directly, they are harvested for two main purposes:

1. Bait for Whelk Fishery:

   • "One of the reasons that horseshoe crabs are harvested is for bait, and that's for the whelk fishery. So a whelk is a giant snail and they...they love the smell of horseshoe crabs, especially horseshoe crab eggs." [24:25 - 24:42]

2. Blood Harvesting:

   • "Horseshoe crab blood has a very special property. They have a really sensitive immune system... a small amount of it can be used to test whether that is sterile." [24:43 - 26:50]

Sarah highlights ongoing efforts to find synthetic alternatives to horseshoe crab blood to prevent overharvesting:

"There's an effort to find a synthetic alternative so that horseshoe crabs don't have to be harvested at all for that purpose." [26:50 - 26:55]

She further emphasizes the need for humane treatment during harvesting and the development of synthetic compounds:

"Researchers are trying to figure out how to make sure the crabs are treated fairly...find ways to make a chemical that does what the horseshoe crab blood does so they don't need to endanger the crabs at all." [27:22 - 27:57]

Unique Features: Blood Color and Properties

A fascinating aspect of horseshoe crabs is their blue blood, explained by Sarah:

"Their blood is blue. Yeah, it's a sort of a sky blue color...because the molecule that's used to carry oxygen in horseshoe crab blood is based on copper. It's called hemocyanin." [27:30 - 28:11]

She contrasts it with human blood, which is red due to iron-based hemoglobin:

"Our blood is red. And that molecule in our blood has iron in the middle. So, Jane, what color does iron turn when it's exposed to oxygen? Rusty red." [28:11 - 28:16]

Further analogies help visualize the color difference:

"So that's why their blood is blue... or the Statue of Liberty." [28:16 - 28:32]

Insights and Conclusions

Sarah Grady and Jane Lindholm conclude the episode by emphasizing the ecological significance of horseshoe crabs. Their eggs are a vital food source for migratory shorebirds, such as the Red Knot, which relies on these eggs to fuel their long journeys from the Arctic to South America.

"Not all of the eggs end up buried under the sand... some of them can provide lots of food for shorebirds, like the Red knot." [18:32 - 19:08]

They also discuss the current status of horseshoe crab populations, which vary by region. Conservation efforts, including prohibiting harvesting during spawning, are crucial for the species' recovery and sustainability.

Jane offers practical advice for young listeners:

"If you're interested in looking closer at a horseshoe crab, it's a good idea to pick it up and get to know it a little bit. Make sure you put it back in the water... hold it like a bowl of cereal in your hand." [15:36 - 16:01]

She encourages participation in local conservation projects, urging children and their families to engage with wildlife organizations to support horseshoe crab preservation.

Notable Quotes

• Jane Lindholm: "Did you know that horseshoe crabs aren't really crabs? I didn't. They're more closely related to spiders and scorpions, as she said. So we've been calling them the wrong thing all along. At least we got the horseshoe part right." [11:05 - 11:32]

• Sarah Grady: "Their blood is blue...because the molecule that's used to carry oxygen in horseshoe crab blood is based on copper. It's called hemocyanin." [27:30 - 28:11]

• Sarah Grady: "Horseshoe crab blood has a very special property. They have a really sensitive immune system... a small amount of it can be used to test whether that is sterile." [24:43 - 26:50]

• Jane Lindholm: "So they're not doing it to be mean." [15:26]

Conclusion

This episode of But Why: A Podcast for Curious Kids offers an engaging and informative exploration into the world of horseshoe crabs. Through vivid descriptions, expert insights from Dr. Sarah Grady, and direct engagement with questions from young listeners, Jane Lindholm effectively demystifies these ancient creatures. The episode not only educates but also inspires young minds to appreciate and participate in wildlife conservation efforts, highlighting the interconnectedness of all species within our ecosystems.